I. Field of the Invention
This invention relates generally to implantable electronic monitoring apparatus, and more particularly an implantable electronic device for monitoring aerobic capacity of patients.
II. Discussion of the Prior Art
A typical prior art pacemaker of the rate adaptive type has commonly incorporated one or more sensors for detecting a physiologic function and for producing an electrical control signal proportional thereto. The electrical control signal is then applied to the timing circuitry of the pacemaker for adjusting the rate at which cardiac stimulating pulses are produced from a programmed lower rate limit to a similarly programmed upper rate limit. Physiologic sensors have included accelerometers for detecting body motion and, in this regard, reference is made to the Meyerson et al. U.S. Pat. No. 5,179,947. Other physiologic parameters that change with hemodynamic need include blood temperature (Cook et al. U.S. Pat. No. 4,436,092), oxygen saturation (Wertzfeld et al. U.S. Pat. No. 4,202,339), the heart's preejection interval (Citak et al. U.S. Pat. No. 4,773,401) as well as respiratory factors including tidal volume (Alt U.S. Pat. No. 4,919,136), respiration rate (Krasner U.S. Pat. No. 3,593,718) and the product thereof, minute ventilation (Plicchi et al. U.S. Pat. No. 4,596,251). These respiratory factors can be derived from the accelerometer output signal using signal processing techniques to isolate components of body motion due to breathing. Alternatively, electrodes may be provided for measuring variations in electrical impedance between electrodes placed in the thoracic cavity.
Implantable cardiac pacemakers are now also being used in treating patients suffering from congestive heart failure (CHF). It has been found that the efficiency of the heart as a pump can be improved by pacing the ventricles with an optimum AV delay between the occurrence of an intrinsic atrial depolarization and the application of a ventricular stimulating pulse.
From the foregoing, it is clear that the technology exists for incorporating within an implantable, fluid impermeable, body compatible housing electronic circuitry including a microprocessor-based controller that is adapted to receive the output from a variety of physiologic sensors including accelerometers and means for sensing respiratory parameters for controlling a pulse generator whose output is used to stimulate the atrium, the ventricles or both.
In addition to the pacing function, implantable cardiac rhythm management devices are increasingly taking advantage of the increase in memory capacity of microprocessors to monitor and store a variety of parameters for later read-out over a telemetry link to an external monitor/programmer module. For example, heart rate variability over a period of many days may be calculated and stored for later read-out. See Heemels et al. U.S. Pat. No. 5,603,331. Heart rate variability has been found to be a significant indicator of the progress of CHF.
Another measure of the efficacy of treatment by drugs, pacing or a combination thereof is the change in aerobic capacity resulting from the therapy. During the recovery stage following exercise, the oxygen consumed, or equivalently the oxygen uptake, is above normal resting levels. The total oxygen consumed from onset of recovery until the pre-exercise level is reached is referred to as the recovery oxygen uptake. There are two components to recovery oxygen uptake: a fast component indicative of a quick drop in elevated oxygen consumption immediately after the cessation of activity, and a slower phase of recovery referred to as the slow component. For a given work level, the recovery oxygen uptake is less for more aerobically fit individuals and the total time of elevated oxygen uptake is an important measure of a patient's level of physical fitness. Recovery oxygen uptake duration may also be evaluated in determining the efficacy of therapy in CHF patients.
It is accordingly a principal purpose of the present invention to provide an implantable electronic device capable of measuring and storing the aerobic capacity of the patient in whom the device is implanted.